[TowerTalk] HAM IV rotor / mast slipping

Thu Apr 5 16:49:43 PDT 2012

On 4/5/2012 4:12 PM, Pete Smith N4ZR wrote:
> K7NV makes the same observation on his web page, Dick, and suggests that
> if you want to pin a mast the hole needs to be drilled very precisely
> and a pin specifically designed for this purpose be used, rather than a
> bolt.

Ideally you polish the bolt or use a pin and ream the hole.  Use the 
largest bolt or pin possible without weakening the mast (more contact 
area.)  It's almost impossible to just "drill" a hole and get the proper 
fit.
If you want maximum strength without danger of damaging the rotator then 
pinning takes a bit of calculating.  The area must also be nearly a 
perfect fit, (even a press fit) and materials of the same hardness 
unless it's a shear pin (then it must be much softer). IF the pin is 
softer than the mast and mount it needs to be enough softer that it will 
not enlarge the hole because it (the pin) will get smaller allowing the 
mast to turn. If it's too hard the opposite will happen and the hole 
will become larger.  Still if the shear pin is too soft it will quickly 
be reduced in size allowing the antennas to swing.  Bolts come, 
ungraded, Grade 5 and grade 8.  I'd not use a grade 8 because they are 
likely to be harder than almost any steel mast you use. They may 
eventually shear, but they will also enlarge the bolt holes in the mast.

Myself, if using a shear pin  I'd prefer a hardened area around the hole 
, or a mast made of very hard material.  OTOH not something that would 
break easily.  Another approach would be to reinforce the mast where its 
pinned by welding in a sleeve, but you do not want to trap water in 
there if it ever freezes in your area.

Best of all would be a mount that was custom fit to the mast with an 
interface material of a rubber like substance that would shear, or lose 
its grip a bit under that where damage would be done to the rotator.  I 
think this would resemble a "Slipp not" with a rubber like lining.  Most 
mounts are "universal mounts" that will fit a range of masts which means 
they fit none of them properly although they may have a gripping force 
appropriate for that particular rotator, but how do you know?  How tight 
should the bolts be torqued? To get the appropriate gripping force they 
must be torqued.  Otherwise it's far too easy to end up with 
insufficient or excess gripping force with no way of knowing which you have.

For other than small tribanders and small UHF or VHF arrays I'd stick 
with the double worm gear drive rotators.  Compared to the Tail Twister 
most are only a little more expensive, but a whole lot more reliable and 
you don't have to worry about the wind during a contest or schedule. 
OTOH don't forget any of them can fail and some of the more expensive 
may not be the best designed or constructed.

If you have to modify the mast mount for any particular rotator, is it 
really in the best interests of that rotator.

73

Roger (K8RI)

>
> 73, Pete N4ZR
> The World Contest Station Database, updated daily at www.conteststations.com
> The Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com,
> spots at telnet.reversebeacon.net, port 7000 and
> arcluster.reversebeacon.net, port 7000
>
>
> On 4/5/2012 2:21 PM, Dick Dievendorff wrote:
>> Someone suggested to me that I might increase the mast to clamp friction by
>> wrapping the mast with something with a little bite.  Do they make
>> double-sided emery paper that can tolerate being wet?
>>
>> I have a clamp to mast size mismatch that I'm going to try to correct with
>> shims, and I might end up pinning too.  My prop pitch doesn't have a wedge
>> to break.
>>
>> I've used pins (usually grade 8 bolts) in the past and they have failed
>> because they weren't hefty enough. The hole ended up larger than the pinning
>> bolt and the wind would work it back and forth enough that it eventually
>> fatigued and failed.  The hole got larger in this process.
>>
>> I think I had too much on the mast.  I've changed that in my new
>> installation.
>>
>> Dick, K6KR
>>
>>
>> -----Original Message-----
>> From: towertalk-bounces at contesting.com
>> [mailto:towertalk-bounces at contesting.com] On Behalf Of Frank
>> Sent: Thursday, April 05, 2012 5:24 AM
>> To: n8de at thepoint.net
>> Cc: KA9S - Jeff; towertalk at contesting.com
>> Subject: Re: [TowerTalk] HAM IV rotor / mast slipping
>>
>> n8de at thepoint.net wrote:
>>
>>> The Tailtwister (big brother of the Ham IV) has a pre-drilled hole
>>> through the upper rotor housing and clamp piece.
>>>
>>> I'd pin the mast with your rotor, just as is done with the T2X.
>>>
>>> 73
>>> Don
>>> N8DE
>>>
>>>
>> Pinning the mast to the rotator is a sure way of preventing mast slippage.
>> It also puts you at risk for rotor destruction if the wind load becomes
>> overly excessive.  Since there is no good way of predicting how strong the
>> wind is going to be here in Texas,  I have preferred not to pin the mast to
>> the rotor (anymore).
>>
>> It is a PIA to have to re-align the directional calibration every so often,
>> but it is an even bigger PIA to replace the rotor after its gears get
>> striped.
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk